Electrical logging of well bores



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United States Patent O M ELECTRICAL LOGGING OF WELL BORES Robert LeeAldexla Canada, Calif., assgnor to Lane- Wells Company, Los Angeles,Calif., a corporation of Delaware Continuation of application Serial No.52,747, October 4, 1943. This application June 16, 1953, Serial No.362,105

14 Claims. (Cl. 324-1) This invention relates in general to electronicswitching devices and more particularly to an electronic square wavegenerator and associated switching apparatus adapted to be usedprincipally in connection with a multiple elecr-ode system of so-calledelectrical coring or electrical logging of well boreholes by meansincluding a plurality of electrodes lowered into a well borehole on amultiple conductor cable in accordance with the general principles andmethod of operation disclosed in the Norelius Patent No. 2,569,867.

This application is a continuation of my copending application SerialNo. 52,747, tiled October 4, 1948.

In such systems of electrical logging, a suitable supply current isusually generated at the surface of the earth and conducted down througha pair of insulated conductors in a multiple conductor cable to a pairof current input electrodes located Within and spaced apart asubstantial distance along the longitudinal axis of the well borehole.These tw-o input electrodes serve to set up an electric field within theformations traversed by the well bore. The resistivity measurements aregenerally made by a separate pair of longitudinally spaced potentialpick-up electrodes positioned within the well bore either above or belowthe before-mentioned current input electrodes, the distance of thespacing between the pick-up electrodes and the adjacent current inputelectrodes being determined by the so-called lateral depth ofpenetration of the resistivity measurements desired.

The potential or current signal picked up from the thus establishedelectric lield within the formations traversed by the well bore by thepotential pick-up electrodes is preferably conducted up themulti-conductor cable through a separate pair of insulated conductorswhich are connected at the surface exterior to the borehole to suitablemeasuring apparatus usually terminating in a recording galvanometer bymeans of which a continuous graphical record of the resistivitymeasurement variations versus depth, of the -formations surrounding theborehole may be made as the electrodes are moved along the length of theborehole.

The input to the before-mentioned current input electrodes has usuallybeen in the form of an alternating current in order to make it possibleto make simultaneous resistivity and natural potential measurements, asis well known in the art. The use of alternating current for theresistivity measurements makes such simultaneous measurements possibleby employing conventional electric filter circuits to segregate theresulting alternating signal picked up by the potential pick-upelectrodes from the unidirectional signal resulting from the naturalformation potential. The use of alternating current also has otherimportant advantages, among them being the elimination of theundesirable effects upon the accuracy of the measurements caused bypolarization on' the surface of the current input and the potentialpick-up electrodes which occurs when unidirectional current is employed.

As hereinbetore mentioned, such electrical logging systems necessitatethe employment of a multi-conductor cable. Such a multi-conductor cablegenerally comprises a plurality of insulated conductors usually hunchedor twisted together and extending throughout the length of the core ofthe cable and surrounded by one or more concentric lays of supportingsteel wire strands. Such conductor cables may take several forms ofconstruction.

A construction heretofore employed in wells of relatively 2,701,334Patented 1, 1955 shallow depth resembles that of a rope and is made upof a plurality of twisted multiple wire strands, each strand containingan insulated conductor wire core. Another type of construction now beingemployed in present day wells of great depth is that known as thereverse c-oncentric cable, and this cable is constructed of a pluralityof concentric, spirally or helically wound single layers of wires, thelay of each layer being reversed with respect to the adjacent layer. Inthis cable a plurality of insulated conductors are located together inparallel or twisted arrangement at the core.

Of the two before-mentioned types of conductor cables the latter has thebest mechanical characteristics for this type of service, but due to theclose bunching and usual lack of shielding between the severalconductors at the core of the cable, the electrical characteristics, inso .far as they relate to interconductor capacity and electricalbalance, have been found to be less desirable than those of thefirst-mentioned type of conductor cable.

However, in any event, with any presently known type of conductor cableit has been found to be impossible by any manufacturing process yetdeveloped, or any other known means, to attain a perfect balance in theelectrical capacity distribution and magnetic coupling between theseveral insulated conductors thus contained within the cable.Accordingly, the result has always been, heretofore, that when analternating current is .introduced into one. pair of the conductors insuch a cable, a spurious alternating current or potential has beenproduced in the other pair or pairs of conductors in the cable as aresult of such electrical unbalance. Thus, when an alternating currentwas applied to the pair of conductors leading -to the input currentelectrodes, a spurious alternating signal was produced in the pair ofconductors leading up from the potential pick-up electrodes which wassuperimposed upon the desired signal received directly by the pick-upelectrodes and delivered to the measuring circuits. An error was thusintroduced into the resistivity measurements. Heretofore in wells ofmodern depth and under conditions where a relatively shallow lateralpenetration of resistivity measureyments was required, thebefore-mentioned spurious signal was tolerated as being of insufficientmagnitude to render the measurements unusable, although introducing arecognized error.

Now, however, as well boreholes, particularly oil wells, are beingdrilled to relatively greater depths necessitating extremely longconductor cables of high strength, and in view of the requirements fordeeper lateral penetration of resistivity measurements and greaterrefinement, accuracy, and detail in these measurements, the spuriouseects of the electrical unbalance between conductors in the cable havegrown to a magnitude which in the absence of suitable correctivemeasures nearly obscures the resistivity measurements sought to beobtained. The usable sensitivity of this method of electrical logging atgreater depths has thus, in eiect, undergone a great reduction.

As disclosed in the before-mentioned Norelius Patent No. 2,569,867, ithas been discovered that the undesirable effects of the electricalunbalance. between the conductors in the conductor cable and at the sametime all eitects of reactance and impedance associ-ated not only withthe conductor cable but also with the logged formations, may beeliminated by employing an alternating input current in the pair ofconductors leading to the input electrodes having a wave form whichincludes at some point a flat or substantially constant current portion,of which a square Wave form is one special example. As a result of thisform of input current, an alternating electric tield of correspondingwave form is established in the surrounding formations between the inputelectrodes. This alternating iield is tested or sampled by means of apair of spaced potential pick-up electrodes located in the borehole ashereinbefore mentioned, the signal received by the pick-up electrodestransmitted up through another pair of conductors in the conductorcable, and then only that part of the resultant returned signal fromwhich have vbeen eliminated all extraneous portions thereof which aredue to or inuenced 'by the electrical unbalance within the conductorcables and reactance or impedance effects of the conductor cable and theformations is utilized for the resistivity measurements.

Heretofore the generation of a square wave signal input to the pair ofinput electrodes for the hereinbefore described purposes h'as`beenaccomplished largely by mechanical switching devices. Similarly thesynchronization and phase control for the means for selecting thatportion of the signal received by the pick-up electrodes which it isdesired,to-utilize for the measurements, excluding thereby allextraneous signals resulting from the electrical unbalance between theconductors within the conduct-or cable and those due to reactance orimpedance eects within the conductor cable and the formations, have beenaccomplished largely by mechanical means.

Such mechanical means and devices have, among a number of disadvantages,the undesirable inclusion of many moving parts subject to wear,susceptible to maladjustment, and productive of noise and vibration.

With the foregoing in view, it is, therefore, an object of thisinvention to furnish an improved square wave generator.

It is another object of this invention to provide an electronic squarewave generator.

It is a further object of this invention to provide an improved squarewave generator and associated switching and timing devices foraccurately discriminating between the desired and undesired portions ofthe signal received by the electrical logging system herein described.

A potential diterence substantially always exists between any electrodeand a remote ground or between any two spaced electrodes in the uid inthe well borehole, known as the so-called spontaneous potential or thenatural potential. Consequently, a unidirectional natural potentialdifference usually exits between the pick-up electrodes of theresistivity measuring circuit, which is conducted through the conductorcable along with the alternating potential to the resistivity measuringcircuit. This unidirectional potential has no effect upon thealternating potential resistivity measuring circuit so long as itremains at a constant value. However, when the electrodes are movedalong the borehole, variations in the natural potential are encounteredwhich appear in the measuring circuit as a varying signal which, byreason of such variation, introduces another error into the alternatingcurrent resistivity measurements.

It is accordingly another object of this invention to eliminate errorswhich would otherwise be introduced into the resistivity measurements bythe effects of the variation of the natural potential difference pickedup by the potential pick-up electrodes and introduced into theresistivity measurement circuits.

It is still another object of this invention to minimize errors in theresistivity measurements which would otherwise be introduced by pick-upfrom the alternating current power source.

These and other objects, advantages, and features of novelty will beevident hereinafter.

In the drawings, which illustrate a preferred embodiment and mode ofoperation of the invention and in which like reference charactersdesignate the same or similar parts throughout the several views:

Figure l is a schematic wiring diagram of the apparatus of theinvention;

Figure l(A) is a graphical illustration of the electricalchairacteristics of a portion of the apparatus of Figure l; an

Figure 2 is a graphical illustration of the general characteristics andphase relationships of the electrical signals or impulses occurring atvarious locations in the electrical circuits of the apparatus.

The apparatus is as follows:

Referring primarily to Figure 1, is a multi-conductor cable extendingfrom a hoist drum 11 located at the earth surface into a well borehole12 and carrying an electrode system assembly E at the lower end thereof.The insulated conductors contained within the cable 10 arediagrammatically illustrated in parallel dotted lines 13, 14, 15 and 16.

The electrode system E may be one of a number of arrangements well knownin the electrical logging art for making lateral resistivitymeasurements, such as. for example, that disclosed in Schlumberger1,819,923 or Bowsky et al. 2,142,555. One conventional arrangement isillustrated in Figure l hereof and comprises an upper pair oflongitudinally spaced input electrodes Ci and Cz and a lower pair oflongitudinally spaced, so-called potential pick-up electrodes P1-and P2,the two pairs being spaced longitudinally from each other and carriedupon an elongated insulating tubular body through which the respectiveconductors extend from the lower end of the conductor cable 10 to theseveral electrodes. The cable conductors 13 and 14 make electricalconnection at their upper ends at the cable drum with slip rings 17 and1S, respectively, mounted upon the drum or drtun shaft for rotationtherewith, and the lower ends of said cable conductors 13 and 14 areconnected respectively to the beforementioned pair of spaced currentinput electrodes Ci and Cz of the electrode system E. Cable conductorsl5 and 16 are connected at their lower ends with the beforementionedpotential pick-up electrodes P1 and Pz, respectively, and at their upperends with another pair of slip rings 21 and 22, respectively, mounted onthe hoist drum 11 or drum shaft together with the before-mentioned sliprings 17 and 18. a

If. desired, the electrode Ci may be electrically connected to the lowerend of the metal sheath of the cable 10, and thus the whole immersedlength of the cable sheath may serve as a current electrode of extensivearea and longitudinal length within the well. Electrical connection ismade between the several electrodes and the formations surrounding theborehole 12 through conductive well fluid such as drilling mudmaintained in the borehole, an upper level of which is illustrated at20, or by means of suitable borehole wall positive contacting means whena relatively non-conducting well uid such as oil is employed, as is wellknown in the art.

A pair of stationary brushes 25 and 26 make sliding electrical contactwith the drum slip rings 17 and 18, respectively, and these brushes areconnected through electrical connections 27 and 28 to the square wavegenerating apparatus and its associated pulsing and switching controlcircuits which supply the square wave alternating current to the inputelectrodesfCi and Cz, as hereinafter more fully described. Another pairof stationary brushes 29 and 30 make sliding electrical contact with thedrum slip rings 21 and 22, respectively, and these brushes are connectedthrough electrical conductors 31 and 32 to the electrical measuringcircuit and electrical log recording apparatus which receives the signalfrom the pick-up electrodes P1 and Pz to be measured, all as more fullydescribed hereinafter.

The before-mentioned measuring circuit and electrical log recordingapparatus is as follows: Conductors 31 and 32 leading from brushes 29and 30 make connection through blocking condenser and conductor 36 andthrough conductor 37, respectively, with the input terminals 33 and 34of a suitable voltage amplifier 38, of conventional design. The twopick-up electrodes P1 and P2 are thus normally connected across theinput of the voltage amplifier from electrode P1 by way of cableconductor 15, slip ring 21, brush 29, conductor 31, condenser 35, andconductor 36 to terminal 33, and thence in return from terminal 34through conductors 37 and 32, brush 30, slip ring 22, cable conductor 16to electrode P2. From the conductors 36 and 37 leading to the amplifier38 a pair of conductors 39 and 40 make electrical connection with thearmature 41 and contactor 42 of an electromagnetically operated relay asillustrated within dotted enclosure 257 and hereinafter referred to as aspike eliminator relay or transient eliminator relay Rs. Theelectromagnet or coil 44 of the relay Rs, when energized with acontrolling, pulsating current, as will be described hereinafter inconnection with the operation, causes the armature 41 to move intointermittent electrical contact with the contact point 42 to close theelectrical circuit between conductors 39 and 40 and thereby periodicallyplace a short-circulating shunt across the voltage amplifier input. Asuitable relay for this purpose is manufactured by Stevens-Arnold, Inc.,and designated Type 172, having a coil resistance of 1400 ohms andadapted to operate on 18 volts. This relay has a time constant ofapproximately .O01 second when operated at this voltage.

The terminals 46 and 47 of the output circuit of thc amplifier 38 areconnected through conductors 49 and 50 to the terminals 51 and 52 of theinput to a low-pass filter 53. This low-pass lter 53 is designed to haveattenuation characteristics in accordance with the graphicalillustration of Figure l(A), as will be more fully described hereinafterin connection with the operation.

The output terminals 55 and 56 of the low-pass filter IHAMMA...

53 are connected through conductors 57 and 58 to the input terminals 59and 60 of a suitable power amplifier 62 of conventional design andpreferably havipg a socalled push-pull output to which the three outputterminals shown at 64, 65, and 66 make connection, the terminal 65 beingthe neutral or center tap connection. The output from the powerlamplifier is fed to a rectifier, preferably a full wave rectifier andpreferably of a mechanical relay type such as that illustrated withinthe dotted enclosure 271 and hereinafter referred to as the rectifierrelay Rr.

Y The output terminals 64 and 66 of the power amplifier are connectedthrough conductors 68 and 69 with opposite stationary contact points 70and 71 of the rectifier relay Rr. An armature 72, spring mounted andresonated to vibrate at approximately the frequency of the poweramplifier output frequency to be rectified, is positioned intermediatethe contact points 70 and 71 and adapted alternately to make electricalcontact with said points 70 and 71 when vibrated by the electromagnet 75energized with a control current of suitable frequency, as describedhereinafter in connection with the operation. The armature 72 of therectifier is connected through conductor 76 to one terminal of a directcurrent meter or galvanometer 77, and the other terminal of thegalvanometer is connected in return through conductor 78 to the centertap connection terminal 65 of the power amplifier 62.

A hand or pointer 80 which is attached to the movement of thegalvanometer 77 carries a pen or other suitable marking device whichbears upon a strip of graph paper 82 adapted to move between rollers 83and 84 for the purpose of tracing a curve or making a graphical recordof the deflection of the galvanometer hand, as illustrated at 85. Therate of motion of the graph or chart paper under the pen from roll 84 toroll 83 is preferably proportional to the rate of motion of theelectrode system E into and out of the well borehole, and thisproportional motion may be accomplished by coupling an idler pulley 84,over which the conductor cable passes, with the paper transportingmechanism, through suitable means such as by a shaft, belt, or the likemechanical device similar to those disclosed in Iakosky Re. 21,797 orElliott 2,222,608, or by electromechanical means as disclosed in Bowsky2,142,555, or as schematically illustrated in Figure l herein, in whicha Selsyn generator 89 is driven through a shaft 90 or other suitablegearing coupled to the idler pulley 87, and this generator in turn isremotely coupled electrically through conductors 91 to a Selsyn motor 92which in turn drives the paper transporting roll 83 through suitablereduction gearing such as a worm gear drive as shown at 94.

The square wave generator circuits and the associated pulsing and timingcontrol circuits and apparatus combined with the electrical logging areas follows:

For convenience of description the several principal unitarysub-assemblies of the circuit, which combine to make up the wholecircuit diagram of the electrical apparatus of Figure l, have been setapart within the several dotted line enclosures shown at 100, 115, 125,150, 170, 192, 193, 194, 195, 225, 257, and 271.

Within the first mentioned dotted line enclosure 100 is a power supplysubstantially of conventional design for supplying the pulsing circuitswith the various required currents and voltages, said supply comprisinga power transformer 101 having a primary winding 102 and three secondarywindings 103, 104, and 105, 103 being a high voltage center tappedwinding, the end connections of which are connected to the anodes of afull wave rectifier tube T1, 104 being a low voltage ftlament currentsupply for the cathode of the rectifier tube T1,

n and 105 representing the winding or windings for supplying filamentcurrent or cathode heater current for all of the other electron tubes inthe circuit, as conventionally indicated.

The rectified current from the cathode of the rectifier tube and thecenter tap of the transformer secondary is passed by way of conductorconnections 106 and 106' through a conventional filter circuit as shownat 107, and the filtered D.C. therefrom applied through conductors 10Sand 109 across a voltage divider comprising a series of resistances 110,111, 112, and 113.

A peak clipper or amplitude limiter circuit is shown within the dottedenclosure 115, comprising two diode electron tubes T2 and T3. The anodeof diode T2 and the cathode of diode T3 are connected through a commonconductor 120 and a pair of resistances 118 and 119 placed in series, tothe end of the transformer secondary winding 103. The cathode of diodeT2 is con nected to the voltage divider at the juncture point betweenresistances 112 and 113, and the anode of diode T3 is connected throughconductor 109 to the center tap connection 106 leading to the center tapof the transformer secondary winding 103. By these connections provisionis made for impressing an alternating potential across diode T3 and thesame alternating potential plus a superimposed undirectional potential,representing the drop across resistance 113, across diode T2 to producein conductor 141 a pulsating unidirectional potential as the output ofthe before-mentioned peak clipper circuit unit 115 as hereinafter morefully described in connection with the operation.

An Eccles-Jordan type trigger circuit is shown within the dottedenclosure 125, which serves as a first stage frequency divider. TheEccles-Jordan frequency divider, trigger circuit is now well known i'nthe art, and more detailed explanations of its circuit and operation maybe found in an article by Eccles and Jordan, Radio Review, vol. 1 1919,page 142, or in several texts such as, for example, Theory andApplications of Electron Tubes by Herbert J. Reich, Ultra-High FrequencyTechniques by Brainerd, Koehler, Reich and Woodruff, page 172, andothers. In brief, the circuit comprises two triode elec tron tubes T4andn T5, the anode of tube T4 being crosscoupled to the grid of tube T5through conductor 130 and resistor 123 shunted by condenser 129, and theanode of tube T5 being likewise cross-coupled to the grid .of tube T4throughV conductor 131 and resistor 132 shunted by a condenser 133.Anode voltage is supplied to the anodes of tubes T4 and T5 from a pointnear the positive end of the voltage divider intermediate resistors and111, through a positive lead conductor 135, and thence through anoderesistors 136 and 137, respectively. The grids of the tubes T4 and T5are connected through grid biasing resistances 138 and 139 to thenegative end of the voltage divider through a negative bus connection144. The cathodes of tubes T4 and T5 are connected to a negative busconnector 140 which leads to the voltage divider at a point betweenresistances 112 and 113 adjacent the negative end of the voltagedivider. The two tubes T4 and T5 are electrically symmetrical withrespect to their control grid biases and anode-cathode and grid voltagessuch that the trigger circuit has two degrees of stability in whicheither tube.

may be conducting and the other non-conducting. For example, if tube T4is conducting, the anode current voltage drop through resistance 136 issufhcient to drive the grid of T5 to cut-oft and at the same time, solong as tube Ts is thus non-conducting, the grid of tube T4 will remainsufiiciently positive to permit continuation of flow of currenttherethrough. The circuit will remain stably in this condition untildisturbed. Likewise, if tube T5 is conducting, the anode current voltagedrop through resistance 137 is sufficient to drive the grid of tube T4to cut-off, and the grid of tube T5 will remain suiciently positive topermit continuation of ow of current therethrough. Upon application of asufficient negative potential pulse to the grids of tubes T4 and Tsthrough the differentiating coupling comprising condensers 142 and 143,that grid of the tube which is conducting at that time will bemomentarily driven more negative, causing a reduction of anode currenttherein. For example, if tube T4, is conducting, the negative pulse willlower the potential of its grid, resulting in a reduction of anodecurrent through resistance 136. The resultant reduction in voltage dropthrough resistances 136 will in turn raise the grid potential of tubeTs, permitting anode current flow through tube T5 and resistance 137.The resultant increase in voltage drop through resistance 137 will inturn further lower the potential of the grid of tube T4. This actionproceeds almost instantly to complete cut-off of tube T4 and saturationcurrent flow through tube T5. Upon application of another negativetriggering pulse, these steps are reversed. resulting in transfer ofconductivity back to tube T4.

The output from the before-mentioned peak clipper or amplitude limitercircuit is coupled to the input of the Eccles-Jordan circuit forsupplying thereto negative potential triggering pulses, as hereinbeforementioned. This coupling comprises a dierentiating impedance networkformed by conductor 141, a pair of differentiating condensers 142 and143, and resistances-139 and 138 through which the anode of diode T2 andcathode of diode T3 are connected with the control grids of tubes T4 andT5.

A multivibrator circuit is shown within the dotted enclosure 150. Thismultivibrator serves as a phase shifting time delay circuit asand forthe purpose described hereinafter in the operation. This type of circuitis well known in the art, and more detailed descriptions thereof may befound in several texts, for example, in Theory and Applications ofElectron Tubes by Reich, page 355. ln brief, the circuit comprises twotriode electron tubes Te and T1, the anode of tube Ts being coupledthrough conductor 155 and condenser 156 to the control grid of tube T7,and the anode of tube T7 likewise being coupled through conductor 157and condenser 158 to the control grid of tube Ts. The grids of tubes Tand T7 are connected through grid biasing resistances 160 and 161 tonegative bus conductor 144 which leads to the negative end of thevoltage divider as before mentioned, and the cathodes of the tubes areconnected to negative bus conncctions 144 and 140 which are in turnconnected to the voltage divider as before described. Voltage issupplied to the anodes of tubes T5 and T1 through positive leadconductor 135 and thence through resistance 162 for tube T7 and throughresistances 163 and 164 for tube Ts. The tubes Ts and T7 are biasedunsymmetrically and operated on different anode-cathode voltages suchthat, normally, in the absence of an initialing negative pulse, tube T6is conducting and tube T1 is non-conducting. This is the stablecondition of the circuit.

Upon application of a negative pulse to the control grid of tube Ts, areduction in anode current therein is caused which increases thepositive potential, or, in other words, reduces the negative bias ontube T7 sufficient to permit anode current ow therein, which in turnreacts upon tube Ts to still further reduce anode current in tube T6until tube TG is non-conducting and tube Tf1 is conducting tosaturation. This is the unstable condition of the circuit. Tube T1 willconduct current for a period of time depending upon the time constant ofcondenser 156 and resistance 161, at the end of which time thebefore-described steps will automatically reverse, and the circuit willip back or restore itself to its original condition of stability. Theoutput from one side of the first stage Eccles-Jordan circuit, 12S, isemployed for supplying negative pulse excitation of the multivibrator150, and this is taken from the anode connection of tube Ts and thencepassed through a differentiating impedance network comprising condenser166, conductor 167, and resistance 160 and applied to the grid ofmultivibrator tube Ts.

Another Eccles-Jordan type trigger circuit, similar to that describedhereinbefore, is shown within the dotted enclosure 170, which serves asa second stage frequency divider. This circuit comprises two triodeelectron tubes T8 and T0, the anode of tube Ta being coupled to the gridof tube T9 through conductor 175 and resistor 176 which is shunted by acondenser 177, and the anode of tube T9 being likewise coupled to thegrid of tube Ta through conductor 178 and resistor 179 which is shuntedby a condenser 180. Anode voltage is supplied to the anodes of tubes Tsand T0 from a point near the positive end of the voltage dividerintermediate resistances 110 and 111, through the positive conductorlead 135 and thence through resistances 181 and 182, respectively. Thegrids of the tubes Ts and Ta are connected through grid biasingresistances 183 and 184 to the negative end of the voltage dividerthrough the negative bus connection 144. The

cathodes of tubes Ts and T9 are connected to negative bus connector 140which, as before mentioned, leads to the voltage divider at a pointbetween resistances 112 and 113 adjacent the negative end thereof.

An output connection is taken at a point between resistors 163 and 164of the anode of tube T0 of the hereinbefore described multivibrator`which leads through conductor 186 and a pair of differentiatingcondensers 187 and 188 to the grids of the Eccles-Jordan tubes Ta andTs.

The output from opposite sides of the Eccles-Jordan circuit areconnected through conductors at 190 and 191. respectively, with theinput circuits of two switching circuits respectively shown within thedotted line enclosures 192 and 193. The switching circuit 192 comprisesa pair of pentode electron tubes T10 and T11, and the beforementionedconnection 190 from the Eccles-Jordan tube Ta leads to the control grids196 and 197 of these tubes.

The switching circuit 193 comprises a pair of pentode electron tubes T12and T13, and the before-mentioned connection 191 from the Eccles-Jordantube Tg similarly makes connection with the control grids 198 and 199 ofthese tubes.

The cathodes of the two pairs of switching tubes of the switchingcircuits 192 and 193 are connected to the negative bus connector 140.All of the cathodes and the grids of the switching tubes are connectedand operate in parallel. The anodes, however, of the pairs of switchlngtubes T10, T11, and T12, T13, lead through separate connections to otherswitching tubes, as hereinafter described.

The screens of the two pairs of switching tubes are connected throughconductors 205 and 206 to a conductor lead 207 which in turn makesconnection with the voltage divider at apoint intermediate the seriesresistances 111 and 112.

The anodes of the two pairs of switching tubes, namely, tubes T10, T11,and T12, T13, are connected through conductors 208, 209, and 210, 211,respectively, and through resistances 212, 213 and 214, 215,respectively, and thence through conductor lead 217 to the positiveterminal of another direct current supply 218. The negative terminal ofthe direct current supply 218 is connected through conductor 220y to thepositive end 108 of the series of voltage divider resistances 110, 111,112, and 113'.

Within the dotted enclosure 22S is a square wave generating or switchingcircuit comprising four triode electron tubes as indicated at T14, T15,T15 and T11.

The anodes of the hereinbefore described switching tubes T10 and T11 areconnected through conductors 208 and 209 and conductors 230 and 231 withthe grids of switching tubes T14 and T15, respectively. Similarly theanodes of Switching tubes T12 and T13 are connected through conductors210 and 211 and conductors 232 and 233 with the grids of switching tubesT16 and T11. respectively. The anodes of switching tubes T15 and T16 areconnected through conductors 235 and 236, respectively, to conductor 217which, as before mentioned, leads to the positive terminal of the directcurrent supply 218. The cathodes of switching tubes T14 and T17 areconnected at 237 and 238 with conductor lead 242, which in turn leads tothe negative terminal of the before-men` tioned direct current supply218.

The cathode of switching tube T15 is connected through cross-connector245 with the anode of switching tube T17, and similarly the cathode ofswitching tube T16 is connected through cross-connector 246 with theanode of switching tube T14.

A pair of conductors 24S and 249 are connected to the cross-connectors245 and 246, respectively, and lead out to the input circuit of theelectrical logging apparatus, as will be more fully describedhereinafter.

Referring now again to the first stage Eccles-Jordan trigger circuitshown within the dotted enclosure 125, an electrical connection is madebetween the grid of triode T5 through conductor 250 and resistor 251 tothe grid of the triode switching tube T18. The cathode of switching tubeT18 is connected through conductor 253 with the negative lead bus 140,and the anode is connected to the positive end 108 of the voltagedivider by Way of conductor 255, field coil 256 of a spike eliminatorrelay illustrated within the dotted enclosure 257, and thence throughresistor 258 and conductors 259 and 260 to the beforementioned positiveend 108 ot' the voltage divider.

Referring again to the second stage Eccles-Jordan switching circuitwithin the dotted line enclosure 170 and the pair of pentode switchingtubes T10 and T11 connected thereto, another switching triode electrontube is shown at T19, the grid of which is connected through conductor266 in common with control grids 196 and 197 of pentodes T10 and T11,respectively, to the grid of the Eccles-Jordan triode tube T0. Thecathode of switching tube T19 is connected through conductor 227 withthe negative lead bus 140. The anode of switching tube T111 makesconnection with the positive end 108 of the beforedescribed voltagedivider by way of conductor 269, field coil 270 of a rectifier relayelectromagnet as illustrated within the dotted enclosure 271, and thencethrough resister 272 and conductor 273.

The operation of the before-described circuits and the apparatus, inconnection with the electrical logging system, is as follows:

The primary 102 of the transformer 101 is supplied with an alternatingcurrent of suitable frequency, preferably relatively low and ofsinusoidal wave form. Although other comparable frequencies may beemployed, for convenience the frequency of 60 cycles per second isassumed to be supplied in connection with the following descriptionofthe operation of the apparatus.

The high voltage from secondary 103 of the transformer 101 is rectifiedand passed through the filter 107, and the resultant unidirectionalpotential applied to the opposite ends of a voltage divider comprisingseries connected resistances 110, 111, 112 and 113 from which thevarious voltage supply connections are made to the electron tubes in thecircuit, as previously described.

Alternating current having a frequency of 60 cycles per second and asinusoidal wave form as illustrated in Figure 2-A is obtained from thelower half portion of the center tapped transformer secondary 103 andapplied through conductors 106' and 109 from one end thereof and throughresistances 118 and 119 from the other end thereof and through conductor120 to the diodes T2 and T3. lt is to be noted that diode T3 with seriesresistances 11S and 119 is connected directly across this alternatingcurrent supply, but diode T2 with series resistances 118 and 119 isconnected across this alternating current supply and the portion 113 ofthe voltage divider resistance. Diode Tz'thus has impressed upon it apredetermined D.-C. component in addition to an A.C. component. Thesediodes are reversed with respect to one another, as shown, so that whenone is conductive the other is inactive, and vice versa, and they thusoperate in a conventional manner in conjunction with the resistances 118and 119 as peak clippers or limiters resulting in a unidirectionalpulsating output potential between conductors 141 and 144 having anapproximately square wave form and of the same frequency as the supplycurrent and appearing approximately as graphically illustrated in Figure2-B.

The unidirectional pulsating potential of the form illustrated in FigureZ-B is applied through 141 to the differentiating condensers 142 and 143with resultant pulsating potentials being simultaneously applied to thegrids of electron tubes T4 and T5 of the lirst stage Eccles-Jordantrigger circuit having the approximate form illustrated in Figure Z-C.such pulsations having a frequency equal to that of the originalalternating current supply or, in the present example, a frequency of 60cycles per second.

In accordance with the well known operating characteristics ot theEccles-Jordan trigger circuit, and as briefly outlined hereinbefore, thecircuit has two degrees of stability, in each of which only one of theelectron tubes T4 or T5 is conducting at a time, the grid of one of thetubes being driven to cut of potential and maintained there by such owof current in the circuit of the opposite tube, and vice versa. Eachtime the grids of the electron tubes T4 or T5 receive a negative pulse,such as one of the negative peaks of the potential wave illustrated inFigure 2-C, the conductivity through the Eccles-Jordan trigger circuitis transferred from one tube to the other. For example, if tube T4happens to be initially conducting and tube Ts in a non-conductingstate, the rst negative pulse impressed upon the grids of the tubes willresult in the substantially instantaneous transfer of conductivity fromtube T4 to tube T5; then upon impression of the second negative pulseupon the tubes, conductivity is again retransferred from tube T to tubeT4, and so on, the conductivity alternating between tubes T4 and T5 solong as negative control pulses are supplied. The result is that for anyone of the two tubes in the Eccles-Jordan trigger circuit the resultantfrequency of its conductivity is half of that of the frequency of theapplied negative control pulses or, in the present assumed case, 30cycles per second. The resultant anode current in one of the tubes, forexample, tube T5, would appear as illustrated in dotted lines at (a) inFigure 2-D, and the corresponding anode voltage would appear asillustrated at (b) and each having a square wave form and a frequency of30 cycles per second. The corresponding grid potential of tube T5 sillustrated at (c).

The thus formed 30 cycle square wave potential (b) from the anodecircuit of the Eccles-Jordan tube Ts is impressed upon thedifferentiating circuit comprising condenser 166 with the resultantdilerentiation thereby to a peaked pulsating alternating current of 3'0cycles per second in conductor 167 and through resistance 160 resultingin corresponding peaked alternating potential pulsations having the formof that graphically illustrated at (d) in Figure Z-E being applied tothe grid of the multivibrator tube Ts. This multivibrator circuitenclosed within the dotted line is biased, as hereinbefore described, isuch a manner that normally when at rest in its stable condition, onlytube T6 is conducting and tube T7 is non-conducting. The negative pulsesapplied to the grid of tube Ts momentarily render is non-conductive,resulting in a temporary transfer of conductance to tube T7 followed byan immediate flip back or return to its stable condition, in whichcondition tube Ts resumes conductivity. The constants of this circuitare such, in the present example, that the duration of time between theapplication of each of the actuating negative pulses and the resultanttransfer of conductance from tube Ts to tube T7 and return isapproximately two milliseconds. The resultant voltage at the anode oftube Ts will appear as graphically illustrated in Figure Z-F, the widthof the square wave potential pulses shown at (e) representing the timeinterval when tube Ts is nonconductive and tube T7 is conducting andhaving a frequency of 30 cycles per second.

The resultant unidirectional potential pulses (e) from the anode of themultivibrator tube Ts are passed through conductor 186 from a pointintermediate the anode resistors 163 and 164 to the differentiatingnetwork hereinbefore described, comprising a pair of differentiatingcondensers 187 and 188 from which connections lead to the grids of theelectron tubes Ta and T9 of the second stage Eccles-Jordan triggercircuit shown within the dotted line enclosure 170. Due to thedifferentiating action of this coupling network, the pulsatingpotentials applied to the before-mentioned grids of tubes Ts and T9 havethe appearance graphically illustrated at (f) and (g) in Figure 2-G, thenegative pulse portions (g) thereof having a frequency of 30 cycles persecond. As before described in connection with the rst stage Eccles-Jordan trigger circuit, at each negative pulse applied to the grids oftubes Ta and T 9 of the second stage Eccles- Jordan trigger circuit, theconductivity is transferred from one tube to the other. The positivepulses have no effect. For example, if tube Ts happens to be initiallyconducting and tube T9 is in a non-conducting state, the first negativepulse impressed upon the grids of the tubes will result in the transferof conductivity from tube Ts to tube T9; then, upon impression of thenext negative pulse upon the tubes, conductivity is again retransferredfrom tube To to tube Ta, and so on, the conductivity alternating betweentubes Ts and T9. The result is that for any one of the two tubes in theEccles-Jordan trigger circuit the resultant frequency of itsconductivity is half of that of the frequency of the applied negativepulses or, under the present assumed conditions, a frequency of l5cycles per second. The resultant anode current wave in one of the tubes,for example, tube Ta, would then appear as illustrated in dotted linesat and the corresponding anode current wave in the tube T9 would appearas illustrated in dotted lines at (l) in Figure 2-I. As a result of theanode current ow in tube Ta and the corresponding voltage drop throughresistor 181, the potential wave at the anode of tube Ta and the gridconnection of tube T9 will have the appearance illustrated respectivelyat (h) in Figure 2-H and at (m) in Figure 2-I. Similarly, when tube T9is conducting and as a result of the voltage drop through resistor 182,the potential at the anode of tube T9 and at the grid connection of tubeTs will have the appearance illustrated respectively at (k) in Figure2-l and at (j) in Figure 2-H. Since the control grids of pentodes T1uand T11 are connected at 190 with the before-mentioned grid connectionto tube Ts, the potentials of said control grids of these pentodes willfollow that of 1') of the grid of tube Ta. Similarly the control grids198 and 199 of pentodes T12 and T13 will follow the potential (k) of thegrid connection 191 of tube T9. This will result in the alternate.parallel flow of current through pentodes T10 and T11 and pentodes T12and T13 in phase with the anode current ow (i) and (l) for tubes Ts andT9, respectively, as illustrated in Figures 2-1 and 2 1, the ow ofpentodes T10 and T11 being 180 out of phase with the flow of pentodesT12 and T13 and at a frequency of l5 cycles per second.

When pentodes T and T11 are thus non-conductive, the grids of switchingtubes T14 and T15, being connected respectively through resistors 212and 213 with the positive lead connection 217 of the direct currentsupply 218, and in the absence of any ow of anode current from tubes T10and T11 through these resistors, are biased positive with respect to thecathodes and hence are fully conductive. At the same time, whilepentodes T10 and T11 are non-conductive, pentodes T12 and To: areconducting, the anode current of tubes T12 and T13 resulting inpotential drops through resistors 214 and 215 sufficient to reduce thepotentials of the grids of switching tubes T16 and T17 to cut-offpotential, thus rendering said switching tubes T and T17 non-conductive.Similarly, when pentodes T10 and T11 are conducting current and pentodesT12 and T13 are non-conductive, switching tubes T16 and T17 are renderedfully conductive, and switching tubes T11 and T15 are cut-olf andnonconductive. ln the first condition, where tubes T14 and T15 areconductive and tubes T15 and T17 are non-conductive, current may flowfrom the D.-C. supply 218 through conductor 242, triode switching tubeT14, crossconnection 246, conductor 249, and out in one direction to theelectrical logging input electrodes C1 and C2, as hereinafter more fullydescribed, and return through conductor 248, cross-connection 245,switching tube T15, anode connection 23S and return to the D.-C. supply218 through conductor 217. Next, when switching tubes T16 and T17 arconductive and switching tubes T14 and T15 are non-conductive, currentsfrom the D.-C. supply 218 may ow through conductor 242, switching tubeT17, cross-connection 245 and conductor 248 out in the oppositedirection to the electrical logging input electrodes and return throughconductor 249, cross-connection 246, switching tube T16, anodeconnection 236 and back to the D.C. supply 218 through conductorv 217.Thus it is seen that an alternating current of square wave form andhaving a frequency of l5 cycles per second is applied to conductors 248and 249 leading to the input electrodes C1 and C2 of the electricallogging device, such applied current having a wave form substantially asgraphically illustrated in solid lines at (n) and (o) in Figure 2-J.

inasmuch as triode tube T19 has its grid connected through conductor 266with the control grids of pentodes T10 and T11, the resultant periods ofconductivity thereof will be coextensive and in synchronism with theperiods of conductivity of the pentode tubes T10 and T11, resulting in apulsating unidirectional current through conductor 269, field coil 270of the electromagnet 75 of the rectifier relay 271 and return throughresistor 272, conductors 273 and 260 to the positive terminal 108 of thevoltage divider. The field coil 270 of the rectifier relay (Rr) willthus be energized in synchronism with the switching action of switchingtubes T10 and T11 or at a frequency of l5 cycles per second, with theresult that the rectifier relay armature 72 will be caused to vibrate atl5 cycles per second, alternately making electrical Contact with point71 and point 7) in such a manner as to accomplish a full waverectification of the alternating current or potential appearing betweenconductors 68 and 78 and 69 and 78, and to be measured as hereinaftermore fully described.

Triode tube T18, having its grid connected through resistor 251 andconductor 250 to the control grid of the Eccles-Jordan tube T5, will berendered alternately conductive and non-conductive in synchronismtherewith and at 30 cycles per second. The resultant unidirectionalpulsating anode current of 30 cycles per second will ow from tube T5through conductor 255, field coil 256 of the electromagnet 44 of thespike eliminator relay (Rs) and return through resistor 258, conductor259, and conductor 260 to the positive end 108 of the voltage divider.The field coil 256 of the electromagnet 44 of the spike eliminator relay(Rs) will thus be energized and deenergized at a frequency of 30 cyclesper second, resulting in periodic movement of the armature 41 intocontact with contactor point 42 in synchronism with the operation of thetirst stage Eccles-Jordan trigger circuit and at a frequency of 30cycles per second, with the result that conductors 36 and 37 leading tothe voltage amplifier 38 will be periodically short-circuited by way ofconductor 40, armature 41, contactor point 42, and conductor 39 thirtytimes a second and during the period 12 represented by the square waveform anode current flo in tube Ts illustrated at (a) in Figure 2-D.

'It is to be noted that as a result of the time delay introduced by themultivibrator circuit and represented by the width of the square wavepulses as illustrated at (e) in Figure 2-F, the phase between the anodecurrent pulses as shown at (a) in Figure 2-D, which operate the spikeeliminator relay Rs, is shifted or displaced in time with respect tothat of the square wave alternating current output from the square wavegenerating switching tubes T11- Tm as illustrated at (n) and (o), forexample, in Figure 2-J, the amount of .shift in the present exampleamounting to .002 second 1n time.

Referring now primarily to the electrical logging apparatus portion ofFigure 1, as hereinbefore described, the square wave alternating currenthaving a frequency of l5 cycles per second is caused to ow from theswitching tubes Tnt-T17, as hereinbefore described, through theconductors 248 and 249 to the stationary brushes 25 and 26 which makesliding contact with drum slip rings 17 and 18, respectively, and fromthere the square wave alternating current flows through conductors 13and 14 in the conductor cable 10 to the input electrodes C1 and Czwithin the liuid in the well borehole 12. As stated before, thepotential applied to the conductors leading to the input electrodes C1and C2 is of a square wave alternating form tending to cause a squarewave alternating current to flow having a form as illustrated by thesolid lines in Figure 2-1, but the resultant current is modified by thereactance and impedance characteristics of the input circuit includingthose of the formations, the major effective portion of which is foundto be in the form of the resistance of and electrical capacity betweenand distributed throughout the conductors 13 and 14 contained in theconductor cable 10. The resultant alternating current appearing at thebottom end of the conductor cable and applied to the formation throughthe input electrodes C1 and C2 is, therefore, of a slightly modifiedsquare wave form which, instead of having an ideally square formcomplying with that of the applied potential and current as shown insolid lines in Figure 2-1 is seen to have the initial portions of thecurrent wave at the point of polarity reversal rounded off slightly in acurve as shown in dotted lines at 295, which curve appears in form torepresent an exponential rate of change of current, the constants ofwhich are determined mainly by the capacity and resistance of thebefore-mentioned conductor cable circuits to which the current is fed.Each half cycle of the approximately square wave thus consists of twoprincipal portions, the initial, curved or transitory portion as shownat 295 beginning at 0 degrees plus .O02 second and persisting through Xdegrees, and following that the steady or constant unidirectionalportion 296 extending throughout the balance of the half cycle from Xdegrees to degrees plus .002 second, as illustrated in Figure 2-1 Sincethe alternating current of the form illustrated at 295 and 296 in Figure2-J is, as before mentioned, applied to the formations between inputelectrodes C1 and Cz, and since the formations are primarily resistivein electrical character, then the resultant alternating potentialreceived between the potential piek-up electrodes P1 and P2 from thesurrounding formations will be of similar form, as shown at 299 and 300,but of substantially reduced magnitude or amplitude, such amplitudebeing as illustrated, for example, at 297 in Figure 2-K.

Due to the coupling between the input pair of conductors 13 and 14 andthe output pair of conductors 15 and 16 leading from the pick-upelectrodes P1 and P2 and up through the conductor cable 10, saidcoupling being principally that due to the capacity unbalance betweenthe conductors, although magnetic coupling may also be present to somedegree, the initial transitory portion 295 of each of the input currentwaves, as described in connection with Figure 2 1, produces a highlypeaked transitory potential across the output conductors 15 and 16 whichhas a typical form and phase relationship approximating that illustratedat 298 in Figure 2-K. This peaked portion 298, when superimposed uponthe pickedup potential wave portion having the form illustrated by thedotted line at 299 and the solid line at 300 and having an amplitude asindicated at 297, results in an alternating potential having an overallresultant wave fo as illustrated in solid lines in Figure 2-K.

The before-described resultant alternating potential having the type orform illustrated in Figure Z-K is transmitted to the input terminals 33and 34 of the voltage amplifier 38 at Ythe earth surface from the pairof cable conductors 15, 16 by `way of the circuit comprising pick-upelectrode P1, cable conductor 15, slip ring 21, brush 29, conductor 31,blocking condenser 35, and conductor 36 to. terminal 33 and return fromterminal 34 through conductor 37 and conductor 32 to brush 30 and thenceto slip ring 22, cable conductor 16 and potential pick-up electrode P2.

As hereinbefore described, conductors 36 and 37 leading to the inputterminals 33 and 34 of the voltage amplifier 38 are shunted by a pair ofconductors 39 and 40 which lead to the contactor point 42 and thearmature 41, respectively, of the spike eliminator relay (Rs). When thespike eliminator relay armature 41 is moved into contact with contactor42 by energization of the electromagnet 44, a short-circuit is therebyplaced between said conductors 36 and 37 leading to the input of thevoltage amplifier 38, thereby rendering the amplifier 38 inactive duringsuch short-circuiting interval. As before described, the field coil 256of the electromagnet 44 of the spike eliminator relay Rs is energizedduring each of the current pulses illustrated at (a) in Figure 2-D whichrepresent the anode current pulsations passed through tube T5. By reasonof the time delay introduced by the multivibrator 150, as beforedescribed, a phase displacement occurs between the said current pulsesillustrated at (a) in Figure 2-D which energize the spike eliminatorrelay and that of the square wave alternating current illustrated at (n)and (o) in Figure 2-1 and the resultant picked-up potential appearing atconductors 36 and 37 in Figure l, as illustrated in Figure 2-K. Thisdisplacement in phase, in the case herein illustrated, amounts to a timeperiod of 2 milliseconds, as hereinbefore mentioned. Thus, by comparingthe phase relationship between the current pulses shown in Figure 2-Dand the picked-up potential wave illustrated in Figure Z-K, it will beseen that the spike eliminator relay Rs is energized approximately 2milliseconds prior to the occurrence of the transient peak illustratedat 298 and after a mechanical delay of approximately one millisecondrequired for the operation of the relay following energization, therelay closes and remains closed for a time interval illustrated at (y)in Figure 2L which includes the time interval during which the transient298 persists, after which, at the end of the current pulse cycleillustrated at (a) in Figure 2-D, the relay coil 44 is deenergized andthe spike eliminator relay Rs opens, restoring the voltage amplifier 38to activity. This action results in only the portion of the picked-upsquare wave alternating potential illustrated in solid lines at (p) inFigure 2-L reaching the input terminals 33 and 34 of the voltageamplifier 38. Thus all of that portion of the alternating potential wavepicked-up by the pick-up electrodes Pi and Pz, whose form is iniiuencedby or is a function of any impedance or reactance characteristics of theconductor cable and the formation being logged, is in effect entirelyeliminated, leaving only that portion of the wave which is of constantvalue and substantially only a function of the resistivity values of thesaid formations to reach the input of the Voltage amplifier 38 and topass therefrom to the measuring circuit. It is, therefore, only thislatter selected portion of the picked-up alternating potential whichreaches the ampliliers and the electrical logging measuring or recordingcircuit.

The output from the power amplifierV 62, which has an amplified modifiedsquare wave form proportional to and in phase with that shown in solidlines at (p) in Figure Z-L, is applied through conductors 68 and 69 tothe opposite contactor points 70 and 71 of the rectilier relay Rr. Sincethe field coil 270 of the electromagnet 75 of the rectifier relay Rr isenergized by means of tube T19 in synchronism with the operation of theswitching tubes producing the initial input square wave, namely, tubesTnt-T17, the armature 72 of the said rectier relay is caused to vibrateat the same frequency and in synchronsm therewith between the contactors70 and 71, thereby resulting in a full-wave rectification of thearnplified alternating current having a form similar to that illustratedin Figure 2-L, to produce a unidirectional pulsating current which flowsthrough conductor 76 to the galvanometer 77 and return through cohductor78 to the center tap connection 65 of the power amplifier 62.

The galvanometer 77 employed as illustrated in Figure 1, under theabove-described operating conditions, is of conventional design andresponsive only to the average value or the direct current equivalent ofthe pulsating unidirectional potential applied to it from the rectifierrelay 271. The galvanometer hand 85, which is provided with a suitablepen or marking device at its end, sweeps from side to side across therecord or graph paper 82 in accordance with the variations in theaverage value of the before-mentioned current applied to it from therectifier 271 and as effected by movement of the electrode system Ethrough the borehole as different formation strata of differentresistivities are encountered therein. As the electrode system is movedthrough the borehole, the motion of the conductor cable 10 istransferred from the pulley 87 through the Selsyn system comprising thegenerator 89 and the motor 92 to the roll 83 and the paper 82, therebyresulting in the forming of the logging curve as illustrated at 85,which is proportional in form to both the resistivity characteristics ofthe tested formations and the longitudinal displacement of theelectrodes within the borehole.

The condenser 35 serves to block or exclude the ow of direct currentinto the input of the amplifier 38 which would result from the so-calledspontaneous or natural potential difference usually existing between thepick-up electrode Pi and P2 in the well borehole. However, as theelectrode system E moves through the well borehole and changes in thepicked-up natural potential occur, such changes in potentialwill causecurrent in effect to pass through the blocking condenser 35 resulting incurrent ow through conductors 36 and 37 into the input of the voltageamplifier 38. Due to the action of the spike eliminator relay Rs, thiscurrent iiow into the voltage amplifier input will be interrupted orshort-circuited at a frequency of 30 cycles per second. Thus, whennatural potentials are undergoing a change, a 30 cycle spurious signalwill be introduced into the input of the voltage amplier 38 which, inthe absence of means to suppress it, would pass through and appear atthe output of the power amplifier 62 as a 30 cycle alternating currentand undergo rectification at rectifier relay Rr and be introduced intothe meter 77, finally appearing as a vibratory error in the amplitude ofthe recorded log curve 85.

To prevent the passage of the spurious 30 cycle signal to the measuringcircuit, as before mentioned, a low pass filter is placed between theoutput of the voltage ampliiier 38 and the input of the power amplifier62, as illustrated at 53. This low pas's lter has built'into itattenuation characteristics such as that illustrated by the curve shownin Figure l-A, in which frequency in cycles per second is plotted as theabscissae against the corresponding dbs of voltage attenuation as theordinates. It is to be noted that, as indicated by this curve, the lowpass filter has a maximum attenuation at 30 cycles per second and aminimum attenuation at 15 cycles per second. Thus, since the square wavealternating current introduced into the input electrodes C1 and C2 andpicked up by the pick-up electrodes Pi and P2 and which finally arrivesat the filter 53 has a frequency of l5 cycles per second, and since theinterfering signal which may be introduced by natural potential changesand interrupted by the spike eliminator relay Rs will have a frequencyof 30 cycles per second, the characteristics of the low pass filter are,therefore, such that the interfering 30 cycle signal will be largelysuppressed while the l5 cycle signal which it is desired to measure willbe permitted to pass relatively freely to the power amplifier and thenceto the measuring apparatus.

Another important feature of this low pass filter having thecharacteristics illustrated by the curve in Figure l-A is that theattenuation for any 60 cycle signal is also relatively high as comparedto the attenuation for the desired l5 cycle signal. This has theadvantage of suppressing any undesired 60 cycle signal which may findits way through or be picked up in any manner from the 60 cyclealternating current source supplied to the primary 102 of the powertransformer 101.

The D.C. supply 218 may be of any suitable type, such as a storagebattery, direct current generator, or a direct current supply systemsimilar to that illustrated within the dotted enclosure 100. It isimportant that the D.-C. supply 218 be capable of delivering a. constantcurrent through the square wave switching tubes Tnt-T17 to the inputelectrodes C1 and C2 and thence to the formations, and to insurethatsuch current be substantially constant, a suitable current regulator(not shown) may be included in the supply system 218 or in the circuitcomprising conductors 217 and 242 leading to the switching tubes Tit-Trr.

As before stated, forcconvenience in description, a sixty cyclealternatihgcurrent power source with conditions and apparatus resultingin a cycle exciting current for the spike eliminator relay Rs and a 15cycle square wave output to the electrologging electrodes has beenassumed. Other frequencies may obviously be employed, such as, forexample, those ranging from approximately 30 to approximately 100 cyclesper second for the power source with corresponding square wave outputfrequencies of from approximately 71/2 to approximately 25 cycles persecond.

The terms measure, measuring or metering the alternating orunidirectional potential, current, or signal, as employed herein in thespecification and claims are n ot to be limited in meaning necessarilyto actual quantitatlve determination of such values in terms of volts,amperes, or the like, but include measuring, indicating, or recordingrelative values or variations therein or suitable functions thereof. Itis obvious that, in a number of places in the apparatusherein-described, twin or multiunit tubes havirig single envelopes maybe substituted for several of the separate electron tubes illustrated.For example, the two diodes T2 and T3 may be replaced by a sultable twindiode electron tube. Likewise, the several pairs of cross-connectedtubes in the several trigger circuits and the pairs of tubes in theswitching circuits may be replaced by single, twin unit tubes. Forexample, tubes T4 and T5 of the trigger circuit 125 may be replaced withasuitable twin triode electron tube.

With the foregoing possibility of employing multi-unit tubes, and forconvenience of description and expression but not by way of limitation,the electron tubes including their necessary and various electrodeelements have been occasionally referred to herein as electron dischargepaths. Such electron discharge paths need not necessarily originate fromseparate cathodes but may, in some cases, as is well known in the art,be emitted from a single or common cathode.

It is to be understood that the foregoing is illustrative only, and thatthe invention is not limited thereby but includes all modificationsthereof within the scope of definition of the appended claims.

What is claimed is:

l. In combination: a first trigger circuit having two electron dischargepaths, a control grid and anode in each path and impedance elementscross-connecting the anode of each path to the control grid of the otherpath such that said first trigger circuit has two degrees of electricalstability; a connection and means for applying periodic negativepotential pulses of a given frequency simultaneously to both of saidcontrol grids for tripping conductivity alternately from one dischargepath to the other; a multivibrator circuit having two electron dischargepaths, a control grid and anode in each path and impedance elementscross-connecting the anode of each path to the control grid of the otherpath and unsymmetrically biased such that said multivibrator circuit hasonly one degree of electrical stability whereby one of said dischargepaths is normally conducting and the other is normally nonconducting;means to regulate the time interval of conductivity of said normallynon-conductive discharge path; a first differentiating circuit forming acoupling between an output electrode of one discharge path of said lirsttrigger circuit and the control grid of one path of the multivibratorcircuit for transferring sharply peaked, short duration potential pulsesthereto for tripping the conductivity from the normally conductingdischarge path momentarily to the normally non-conducting discharge pathand return once for each such pulse; a second trigger circuit like therst trigger circuit; a second differentiating circuit forming a couplingbetween an output electrode of said normally conducting discharge pathof said multivibrator circuit and both of the control grids of saidsecond trigger circuit for applying periodic negative pulsessimultaneously to both of the control grids thereof for trippingconductivity alternately from one discharge path to the other, each suchnegative pulse occurring during each triggered cycle of operation at theinstant of each return of conductivity to the normally conductingdischarge path of said multivibrator; and a connection from an outputelectrode of a discharge path of said second trigger circuit whereby apulsating potential may be obtained therefrom having a frequencyone-fourth that of said first-mentioned given frequency and displacedtherefrom in phase by a time interval equal to the time duration ofnon-conductivity of the normally conducting discharge path of saidmultivibrator.

2. In combination: a tirst trigger circuit having two electron dischargepaths, a control grid and anode in each path and impedance elementscross-connecting the anode of each path to the control grid of the otherpath such that said first trigger circuit has two degrees of electricalstability; a connection and means for applying periodic negativepotential pulses of a given frequency simultaneously to both of saidcontrol grids for tripping conductivity alternately from one dischargepath to the other; a multivibrator circuit having two electron dischargepaths, a control grid and anode in each path and impedance elementscross-connecting the anode of each path to the control grid of the otherpath and unsymmetrically biased such that said multivibrator circuit hasonly one degree of electrical stability whereby one of said dischargepaths is normally conducting and the other is normally nonconducting;means to regulate the time interval of conductivity of said normallynon-conductive discharge path; a first differentiating circuit forming acoupling between an output electrode of one discharge path of said rsttrigger circuit and the control grid of the normally conductingdischarge path of the multivibrator circuit for transferring sharplypeaked` short duration negative potential pulses thereto for trippingthe conductivity from said normally conducting discharge pathmomentarily to the normally non-conducting discharge path and returnonce for each such pulse; a second trigger circuit like the firsttrigger circuit; a second differentiating circuit forming a couplingbetween an output electrode of said normally conducting discharge pathof said multivibrator circuit and both of the control grids of saidsecond trigger circuit for applying periodic negative pulsessimultaneously to both of the con trol grids thereof for trippingconductivity alternately from one discharge path to the other. each suchnegative pulse occurring during each triggered cycle of operation at theinstant of each return of conductivity to the normally conductingdischarge path of said multivibrator; and a connection from an outputelectrode of a discharge path of said second trigger circuit whereby apulsating potential may be obtained therefrom having a frequencyone-fourth that of said tirst-mentioned given freouency and displacedtherefrom in phase by a time interval equal to the time duration ofnon-conductivity of the normally conducting discharge path of saidmultivibrator.

3. ln combination: a rst trigger circuit having two electron dischargepaths, a control grid and anode in each path and impedance elementscross-connecting the anode of each path to the control grid of the otherpath snch that said first trigger circuit has two degrees of electricalstability; a connection and means for applying periodic negativepotential pulses of a given frequency simultaneously to both of saidcontrol grids for tripping conductivityI alternately from one dischargepath to the other: a multivibrator circuit having two electron dischargepaths. a control grid and anode in each path and impedance elementscross-connecting the anode of each path tn the control grid of the otherpath and unsymmetrically biased such that said multivibrator circuit hasonly one degree of electrical stability and a given time constant; a rsfdifferentiating circuit forming a coupling between nn output electrodeof one discharge path of said first trigger circuit and the control gridof the normally conducting discharge path of the multivibrator circuitfor transferring sharply peaked short duration negative potential pulsesthereto for tripping the conductivity from said normally conductingdischarge path momentarily to the normally non-conducting discharge pathand return once for each such pulse; a second trigger circuit like thefirst trigger circuit: a second differentiating circuit forming acoupling between an output electrode of said normally conductingdischarge path of said multivibrator circuit and both of the controlgrids of said second trigger circuit for applying periodic positivepulses simultaneously to both of the control grids thereof for trippingconductivity alternately from one discharge path to the other, each suchnegative pulse occurring during each triggered cycle of operation at thet i l instant of each return of conductivity to the normally conductingdischarge path of said multivibrator; a first pair and a second pair ofindependent electron discharge paths with each of said pairs having afirst and second discharge path; a cathode, control electrode and outputelectrode in each of said first and second discharge paths; a connectioncoupling an output electrode of one of the discharge paths of saidsecond trigger circuit to the control electrode of said first pair ofpaths; a connection coupling an output electrode of the Qtherrof thedischarge paths ot' said second trigger circuit to the controlelectrodes of said second pair of paths; a cross-connection from anoutput electrode of the first one of said first pair of paths to thecathode or the first one of said second pair of paths; across-connection from the output electrode of the second one of saidsecond pair of paths to the cathode of the second one of the said firstpair of paths; a source of direct current; a connection from thenegative side of said source to the cathode of said first one of saidfirst and to the cathode of said second one of said second pair ofpaths; a connection from the positive side of said source to the outputelectrode of said second one of said first and to the output electrodeof said first one of said second pair ot' paths; and a pair of outputconnections, one leading from one of said cross-connections and theother leading from the other of said cross-connections from which may beobtained an alternating signal having a substantially square wave formand a frequency one-fourth that of said firstmentioned given frequencyand displaced in phase by a time interval equal to the time duration ofconductivity of the normally non-conducting discharge path of saidmultivibrator.

4. In combination: a trigger circuit having two electron dischargepaths, a control grid and anode in each path and impedance elementscross-connecting the anode of each path to the control grid of the otherpath such that said trigger circuit has two degrees of electricalstability; a connection and means for applying periodic negativepotential pulses of a given frequency simultaneously to both of saidcontrol grids for tripping conductivity alternately from one dischargepath to the other; a first pair and a second pair of independentelectron discharge paths with each of said pairs having a first andsecond discharge path; a cathode, control electrode and output electrodein each of said rst and second discharge paths;.a connection coupling anoutput electrode of one of the discharge paths of said trigger circuitto the control electrode of said first pair of paths; a connectioncoupling an output electrode of the other of the discharge paths of saidtrigger circuit to the control electrodes of said second pair ot paths;a cross-connection from an output electrode of the first one of saidfirst pair of paths to the cathode of the first one of said second pairof paths; a cross-connection from the output electrode of the second oneof said second pair of paths to the cathode of the second one of thesaid first pair of paths; a source of direct current; a connection fromthe negative side of said source to the cathode of said first one ofsaid first and to the cathode of said second one of said second pairs ofpaths; a connection from the positive side of said source to the outputelectrode of said second one of said first and to the output electrodeof said first one of said second pair of paths; and a pair of outputconnections, one leading from one of said crossconnections and the otherleading from the other of said cross-connections from which may beobtained an alternating signal having a substantially square wave formand a frequency one-half that of said first-mentioned given frequency.

5. In combination: a first pair and a second pair of independentdischarge paths with each of said pairs having a first and seconddischarge path; a cathode, control electrode and an output electrode ineach of said discharge paths; a cross-connection between the outputelectrode or the first one of said first pair of paths and the cathodeof the first one of said second pair of paths; a cross-connectionbetween the output electrode of the second one of said second pair ofpaths and the cathode of the second one of the said first pair of paths;a source of direct current; a connection from the negative side of saidsource to the cathode of said first one of said first and to the cathodeof said second one of said second pair of paths; a connection from thepositive side of said source to the output electrode of said second oneof said first and to the output electrode of said rst one of said secondpairs of paths; a pair of output connections, one leading from one ofsaid cross-connections and the other *leading from the other of saidcross-connections; a source of negative potential pulsations; means toapply said pulsations alter nately to the control electrones or saidtirst pair of paths and to the control electrodes of said second pair ofpaths, whereby an alternating signal may be obtained from said outputconnections.

6. ln combination: a first pair and a second pair of independentdischarge paths with each of said pairs having a first and seconddischarge path; a cathode, control electrode and an output electrode ineach of said discharge paths; a cross-connection between the outputelectrode of the first one of said first pair of paths and the cathodeof the first one of said second pair of paths; a cross-connectionbetween the output electrode of the second one of said second pair ofpaths and the cathode of the second one of the said first pair of paths;a source of direct current; a connection from the negative side of saidsource to the cathode of said first one of said iirst and to the cathodeof said second one of said scond pairs of paths; a connection from thepositive side of said source to the output electrode of said second oneof said first and to the output electrode of said first one of saidsecond pairs of paths; a pair of output connections, one leading fromone of said cross-connections and the other leading from the other ofsaid cross-connections; a source of negative potential pulsations ofsubstantially square wave torm; means to apply said pulsationsalternately to the control electrodes or' said lirst pair of paths andto the control electrodes of said second pair of paths, said potentialof' said pulses being suflicient to drive said control electrodes to thecui-oft` potential of their respective discharge paths, whereby analternating signal having a substantially square wave form may beobtained from said output connections.

7. ln electrical logging apparatus wherein a pulsating current ofsubstantially square wave form is conducted through an input conductorcontained in a conductor cable to an input current electrode in aborehole to establish a pulsating electric field of approximately squarewave form in the surrounding formations around said electrode andwherein a portion of said pulsating electric field is tested by a pairot' spaced potential pick-up electrodes in said borehole and theapproximately square wave signal thus picked up by said pick-upelectrodes is conducted through a pair of conductors contained in saidconductor cable to an electric metering device at the surface, cxteriorto said borehole, the apparatus comprising: a rst trigger circuit havingtwo electron discharge paths, a control grid, an anode in each path andimpedance elements cross-connecting the anode of each path to thecontrol grid of the other path such that said first trigger circuit hastwo degrees ot' electrical stability; a connection and means forapplying periodic, negative potential pulses of a given frequencysimultaneously to both of said control grids tor tripping conductivityalternately from one discharge path to the other; a multivibratorcircuit having two electron discharge paths, a control grid and anode ineach path and impedance elements cross-connecting each anode of eachpath to the control grid of the other path and unsymmetrically biasedsuch that said multivibrator circuit has only one degree of electricstability and a given time constant; a first differentiating circuitforming a coupling between said output electrode of one discharge pathof said first trigger circuit and the control grid of the normallyconducting discharge path of the multivibrator circuit for transferringsharply-peaked, short duration nega` tive potential pulses thereto fortripping the conductivity from said normally conducting discharge pathmomentarily to the normally non-conducting discharge path and returnonce for each such pulse; a second trigger circuit like the'firsttrigger circuit; a second differentiating circuit forming a couplingbetween an output electrode of said normally conducting discharge pathof said multivibrator circuit and both of the control grids of saidsecond trigger circuit for applying periodic negative pulsessimultaneously to both of the control grids thereof for trippingconductivity alternating from one discharge path thereof to the other,each such negative pulse occurring during each triggered cycle ofoperation at the instant of each return of conductivity to the normallyconducting discharge path of said multivibrator; a first pair and asecond pair of independent electron discharge paths with each of saidpairs having a first and second discharge path; a cathode, controlelectrode and output electrode in each of said first and seconddischarge paths; a connection coupling an 19 output electrode of one ofthe discharge paths of said second trigger circuit to the controlelectrode of said first pair of paths; a connection coupling an outputelectrode of the other of the discharge paths of said second triggercircuit to the control electrodes of said second pair of paths; across-connection from an. output electrode of the first one of saidfirst pair of paths to the cathode of the first one of said second pairof paths; a cross-connection from the output elecsrodeof the second oneof said second pair of paths to the cathode of the second one of thesaid first pair of paths; a source of direct current; a connection fromthe negative side of said source to the cathode of said first one ofsaid first and to the cathode of said second one of said second pair ofpaths; a connection from the positive side of said source to the outputelectrode of said second one of said first and to the output electrodeof said first one of said second pair of paths; a pair of outputconnections, one leading from one of said cross-connections and theother leading from the other of said crossconnections from which may beobtained a pulsating current of substantially square wave form and afrequency one-fourth that of said first-mentioned given frequency anddisplaced in phase therefrom by a time interval equal to the timeduration of conductivity of the normally nonconducting discharge path ofsaid multivibrator; a connection from one of said output connections tosaid input conductor contained in said conductor cable; a connectionfrom the other of said output connections to a grounded electrodewhereby said pulsating current of substantially square wave form isconducted through said input electrodes and said electric field isestablished in the surrounding formations: an electric metering device;electrical connections to said electrical metering device from said pairof conductors contained in said conductor cable leading from saidpick-up electrodes; switching means associated with said electricalconnections. intermediate the said pair of conductors in said cable andsaid electric metering device for control of the time and duration ofthe application of the said pulsating signal from said pair ofconductors from said pick-up electrodes to said electric meteringdevice, said switching means being electrically coupled to andsynchronously actuated by the output from an output electrode of one ofsaid discharge paths of said first trigger circuit, whereby thepulsating signal reaching said meter is interrupted at an interruptionfrequency twice that of the frequency of said pulsating signal anddisplaced therefrom in phase by a time interval equal to the timeduration of non-conductivitv of the normally conducting discharge pathof said multivibrator.

8. ln an electrical logging apparatus wherein an alternating current ofsubstantially square wave form is conducted through an input conductorcontained in a conductor cable to an input current electrode in aborehole to establish an alternating electric field of approximatelysquare wave form in the surrounding formations around said electrode andwherein a portion of said alternating electric field is tested bv a pairof spaced potential pick-up electrodes in said borehole and theapproximately square wave signal thus picked up by said pick-upelectrodes is conducted through a pair of conductors contained in saidconductor cable to an electric metering device at the surface, exteriorto said borehole` the apparatus comprising: a first trigger circuithaving two electron discharge paths, a control grid` an anode in eachpath and impedance elements cross-connecting the anode of each path tothe control grid of the other path such that said first trigger circuithas two degrees of electrical stability: a connection and means forapplying periodic` negative potential pulses of a given frequencysimultaneously to both of said control grids for tripping conductivityalternately from one discharge path to the other: a multivibratorcircuit having two electron discharge paths, a control grid and anode ineach path and impedance elements cross-connecting each anode of eachpath to the control grid of the other path and unsymmetrically biasedsuch that said multivibrator circuit has only one degree of electricstability and a given time constant: a first differentiating circuitforming a coupling between an output electrode of one discharge path ofsaid first trigger circuit and the control grid of the normallyconducting discharge path of the multivibrator circuit for transferringsharply-peaked, short duration negative potential pulses thereto fortripping the conductivity from said normally conducting discharge pathmomentarily to the normally non-conducting discharge path and returnonce for each such pulse; a second trigger cir- Vayunas-r cuit like thefirst trigger circuit; a second differentiating circuit forming acoupling between an output electrode of said normally conductingdischarge path of said multivibrator circuit and both of the controlgrids of said second trigger circuit for applying periodic negativepulses simultaneously to both of the control grids thereof for trippingconductivity alternately from one discharge path thereof to the other,each such negative pulse occurring during each triggered cycle ofoperation at the instant of each return of conductivity to the normallyconducting discharge path of said multivibrator; a first pair and asecond pair of independent electron discharge paths with each of saidpairs having a first and second discharge path; a cathode, controlelectrode and output electrode in each of said first and seconddischarge paths; a connection coupling an output electrode of one of thedischarge paths of said second trigger circuit to the control electrodeof said first pair of paths; a connection coupling an output electrodeof the other of the discharge patl of said secondtrigger circuit to thecontrol electrodes of said second pair of paths; a cross-connection froman output electrode of the first one of said first pair of paths to thecathode of the first one of said second pair of paths; a cross-connec-`tion from the output electrode of the second one of said second pair ofpaths to the cathode of the second one of the said first pair of paths;a source of direct current; a connection from the negative side of saidsource to the cathode of said first one of said first and to the cathodeof said second one of said second pair of paths; a connection from thepositive side of said source to the output electrode of said second o neof said first and to the output electrode of said first one of saidsecond pair of paths; a pair of output connections, one leading from oneof said cross-connections and the other leading from the other of saidcross-connections from which may be obtained an alternating current ofsubstantially square wave form and a frequency one-fourth that of saidfirst-mentioned given frequency and displaced in phase therefrom by atime interval equal to the time duration of conductivity of the normallynon-conducting discharge path of said multivibrator; a connection fromone of said output connections to said input conductor contained in saidconductor cable; a connection from the other of said output connectionsto a grounded electrode whereby said alternating current ofsubstantially square wave form is conducted through said input electrodeand said electric field is established in the surrounding formations; anelectric metering device; electrical connections to said electricalmetering device from said pair of conductors contained in said conductorcable leading from said pick-up electrodes; switching means associatedwith said electrical connections intermediate the said pair ofconductors in said cable and said electric metering device for controlof the time and duration of the application of the said alternatingsignal from said pair of conductors from said pick-un electrodes to saidelectric metering device. said switching means being electricallycoupled to and synchronously actuated by the output from an outputelectrode of one of said discharge paths of said first trigger circuit,whereby the alternating signal reaching said meter is interrupted at aninterruption frequency twice that of the frequency of said alternatingsignal and displaced therefrom in phase by a time interval equal to thetime duration of non-conductivity of the normally conducting dischargepath of said multivibrator.

9. In electrical logging apparatus wherein a current is conductedthrough an input conductor contained in a conductor cable to an inputcurrent electrode in a borehole to establish an electric field insurrounding formations around said electrode and wherein a portion ofsuch electric field is tested by a pair of spaced potential pick-upelectrodes in said borehole and the potential thus picked up isconducted through a pair of conductors contained in said conductor cableto measuring apparatus at the surface exterior to said borehole,apparatus comprising: means for generating an electric potential havingan alternating potential component of a given frequency and asubstantially square wave form; means for applying said generatedpotential to said conductor leading to said input electrode to therebyestablish an electric field in the surrounding formations having arialternating potential component of said frequency and an approximatelysquare wave form; means for picking up a potential corresponding to aportion of said electric field together with any natural potentialexisting at said potential pick-up electrodes; means for applying thethus picked-up potentials to the said pair of conductors; receivingmeans for receiving from said pair of conductors at the c arth surfaceexterior to the borehole a pulsating potential comprising said picked-upalternating potential component and natural potentials together withsuperimposed transients produced in said pair of conductors; means forinterrupting said pulsating potential received by said receiving means,at an interruption frequency twice that of said generated alternatingpotential component and with each such interruption being so phased andfor such a time interval as to be inclusive of that portion of each halfcycle of said pulsating potential received by said receiving means fromsaid pair of conductors, containing said transients, to obtain therefroma resultant interrupted pulsating signal free from such transients butcontaining signal components having both said interruption frequency andsaid generated alternating potential component frequency; and meteringapparatus, relatively insensitive to said interrupted frequency, formeasuring the said resultant pulsating signal.

10. In electrical logging apparatus wherein a current is conductedthrough an input conductor contained in a conductor cable to an inputcurrent electrode in a borehole to establish an electric field insurrounding formations around said electrode and wherein a portion ofsuch electric field is tested by a pair of spaced potential pick-upelectrode in said borehole and the potential thus picked up is conductedthrough a pair of conductors contained in said conductor cable tomeasuring apparatus at the surface exterior to said borehole, apparatuscomprising: means for generating an electric potential having analternating potential component of a given frequency and a substantiallysquare wave form; means for applying said generated potential to saidconductor leading to said input electrode to thereby establish anelectric field in the surrounding formations having an alternatingpotential component of said frequency and an approximatelysquare waveform; means for picking up a potential corresponding to a portion ofsaid electric eld together with any natural potential existing at saidpotential pick-up electrodes; means for applying the thus picked-uppotentials to the said pair of conductors; receiving means for receivingfrom said pair of conductors at the earth surface exterior to theborehole a pulsating potential comprising said picked-up alternatingpotential component and natural potentials together with superimposedtransients produced in said pair of conductors; means for interruptingsaid pulsating potential received by said receiving means, at aninterruption frequency twice that of said generated alternatingpotential component and with each such interruption being so phased andfor such a time interval as to be inclusive of that portion of each halfcycle of said pulsating potential received by said receiving means fromsaid pair of conductors, containing' said transients, to obtaintherefrom a resultant interrupted pulsating signal free from suchtransients but containing signal components having both saidinterruption frequency and said generated alternating potentialcomponent frequency; a low pass filter having a relatively highattenuation at said interruption frequency as compared to that at thefrequency of said alternating potential component; means for subjectingsaid resultant pulsating signal to the action of said filter; and meansfor measuring the modified pulsating signal resulting from said actionof said filter.

l1. In electrical logging apparatus wherein a current is conductedthrough an input conductor contained in a conductor cable to an inputcurrent electrode in a borehole to establish an electric field insurrounding formations around said electrode and wherein a portion ofsuch electric field is tested by a pair of spaced potential pick-upelectrodes in said borehole and the potential thus picked up isconducted through a pair of conductors contained in said conductor cableto measuring apparatus at the surface exterior to said borehole,apparatus comprising: means for generating an electric potential havingan alternating potential component of a wave form including a.substantially constant amplitude portion in the cycle; means forapplying said generated potential to said conductor leading to saidinput electrode to thereby establish an electric field in thesurrounding formations having an alternative potential component of saidfrequency and having a wave form including an approximately constantampltiude portion in the cycle; means for picking up a potentialcorresponding to a portion of said electric field together with anyunidirectional natural potential existing at said potential pick-upelectrodes; means for applying the thus picked-up potentials to saidpair of conductors; receiving means for receiving from said pair ofconductors at the earth surface exterior to the borehole a pulsatingpotential comprising said picked-up alternating potential component andunidirectional natural potentials together with superimposed transientsproduced in said pair of conductors; means for interrupting saidpulsating potential received by said receiving means, at an interruptionfrequency twice that of said generated alternating potential componentand with each such interruption being so phased and for such a timeinterval as to be inclusive of that portion of each cycle of saidpulsating potential received by said receiving means from said pair ofconductors containing said transients, but exclusive of a portionthereof containing a substantially constant amplitude potential portion,to obtain therefrom a resulant inerrupted pulsating signal ofapproximately square wave form, free from such transients but containingsignal components having both said interruption frequency and saidgenerated alternating potential component frequency; and meteringapparatus, relatively insensitive to said interruption frequency, formeasuring the said resultant pulsating signal.

12. In electrical logging apparatus wherein a current is conductedthrough an input conductor contained in a conductor cable to an inputcurrent electrode in a borehole to establish an electric field insurrounding formations around said electrode and wherein a portion ofsuch electric field is tested by a pair of spaced potential pick-up,electrodes in said borehole and the potential thus picked up isconducted through a pair of conductors contained in said conductor cableto measuring apparatus at the surface exterior to said borehole,apparatus comprising: means for generating an electric potential havingan alternating potential component of a wave form including asubstantially constant amplitude portion in the cycle; means forapplying said generated potential to said conductor leading to saidinput electrode to thereby establish an electric field in thesurrounding formations having an alternating potential component of saidfrequency and having a wave form including an approximately constantamplitude portion in the cycle; means for picking up a potentialcorresponding to a portion of said electric field together with anyunidirectional natural potential existing at said potential pick-upelectrodes; means for applying the thus picked-up potentials to saidpair of conductors; receiving means for receiving from said pair ofconductors at the earth surface exterior to the borehole a pulsatingpotential comprising said picked-up alternating potential component andunidirectional natural potential together with superimposed transientsproduced in said pair of conductors; means for interrupting saidpulsating potential received by said receiving means, at an interruptionfrequency twice that of said generated alternating potential componentand with each such interruption being so phased and for such a timeinterval as to be inclusive of that portion of each cycle of saidpulsating potential received by said receiving means from said pair ofconductors containing said transients, but exclusive of a portionthereof containing a substantially constant amplitude potential portion,to obtain therefrom a resultant interrupted pulsating signal ofapproximately square wave form, free from such transients but containingsignal components having both said interruption frequency and saidgenerated alternating potential component frequency; a low pass filterhaving a relatively high attenuation at said interruption frequency ascompared to that at the frequency of said alternating potentialcomponent; means for subiecting said resultant pulsating signal to theaction of said filter; and means for measuring the modified pulsatingsignal resulting from said action of said filter.

13. In electrical logging apparatus wherein an alternating current isconducted through an input conductor coritained in a conductor cable toan input current electrode in a borehole to establish an electric fieldin surrounding formations around said electrode and wherein a portion ofsuch electric field is tested by a pair of spaced potential pick-upelectrodes in said borehole and the potential thus picked up isconducted through a pair of conductors contained in said conductor cableto measuring apparatus at the earth surface exterior to said borehole,apparatus comprising: means for generating an alternating electricpotential having a given frequency and a square wave 23 form; means forapplying said generated alternating potential to said conductor leadingto said input electrode to thereby establish an alternating electricfield in the surrounding formations having said frequency and anapproximately square wave form; means for picking up a potentialcorresponding to a portion of said alternating electric field togetherwith any natural unidirectional potential existing at said potentialpick-up electrodes; means for applying the thus picked-up potentials tothe said pair of conductors; receiving means for receiving from saidpair of conductors at the earth surface exterior to the borehole apulsating potential comprising said picked-up alternating and naturalpotentials together with superimposed transients produced in said pairof conductors; means for interrupting said potentials received by saidreceiving means, at an interruption frequency twice that of the saidgenerated alternating potential and with each said interruption being sophased and for such a time interval as to be inclusive of that portionof each half cycle of said picked-up potential containing saidtransients, to obtain therefrom a resultant interrupted pulsating signalfree of such transients but containing components having both saidinterruption frequency and said generated alternating potentialfrequency; and metering apparatus, relatively insensitive to saidinterruption frequency, for measuring the said resultant pulsatingsignal.

14. In electrical logging apparatus wherein an alternating current isconducted through an input conductor contained in a conductor cable toan input current electrode in a borehole to establish an electric fieldin surrounding formations around said electrode and wherein a portion ofsuch electric eld is tested by a pair of spaced potential pick-upelectrodes in said borehole and the potential thus picked up isconducted through a pair of conductors contained in said conductor cableto measuring apparatus at the earth surface exterior to said borehole,apparatus comprising: means for generating an alternating electricpotential having a given frequency and a square wave form; means forapplying said generated alternating potential to said conductor leadingto said input electrode to thereby establish an alternating electricfield in the surrounding formations having said frequency and anapproximately square wave form; means for picking up a potentialcorresponding to a portion of said alternating electric field togetherwith any natural unidirectional potential existing at said potentialpick-up electrodes;V

means for applying the thus picked-up potentials to the said pair ofconductors; receiving means for receiving from said pair of conductorsat the earth surface exterior to the borehole a pulsating potentialcomprising said picked-up alternating and natural potentials togetherwith superimposed transients produced in said pair of conductors; meansfor interrupting said potentials received by said receiving means, at aninterruption frequency twice that of the said generated alternatingpotential and with each said interruption being so phased and for such atime interval as to be inclusive of that portion of each half cycle ofsaid picked-up potential containing said transients, to obtain therefroma resuktant interrupted pulsating signal free of such transients butcontaining components having both said interruption frequency and saidgenerated alternating potential frequency; a low pass filter having arelatively high attenuation at said interruption frequency as comparedto the attenuation thereof at the frequency of said generated electricpotential; means for subjecting said resultant pulsating signal to theaction of said filter; and means for measuring the modified pulsatingsignal resulting from said action of said filter.

References Cited in the tile of this patent UNITED STATES PATENTS1,826,961 Slichter Oct. 13, 1931 2,046,436 Waschek July 7, 19362,324,797 Norton July 20, 1943 2,363,987 Muzzey Nov. 28, 1944 2,398,761Aiken Apr. 23, 1946 2,409,689 Morton Oct. 22, 1946 2,420,200 SchoenfeldMay 6, 1947

